It has been known to use a collimator to provide a test pattern using collimated light for the purpose of calibrating optical devices. These collimators have a number of disadvantages, including large size, high power consumption, high heat dissipation, and high cost of operation.
It has been proposed to include a ground glass plate to diffuse light in a collimation system. Such a diffusing means has the disadvantage of scattering light at all angles, so it reduces optical efficiency. What is need instead is a diffusing means that scatters light across a narrow range of angles, not at all angles.
Collimators have used extended illumination sources, such as the output of an integrating sphere. This approach is inefficient. The diverging output from the exit aperture of the integrating sphere must be captured and projected into the exit pupil of the projection lens. Light that exits the integrating sphere outside the numerical aperture of the condensing lens does not contribute to the useful signal. Also, more condensing lens elements are required to convert the diverging output from the integrating sphere into a converging beam. The over-all length, mass, and cost of this technique are high. What is needed is a light source that has compact length, is not heavy, and is relatively inexpensive.
It has also been proposed to use in a collimator an illumination source with one or more tungsten-halogen lamps. These lamps require high voltages to operate. Their efficiency, defined as the ratio of optical power within the spectral band divided by wallplug electrical power consumption, is far lower than that of off-the-shelf light emitting diodes. Tungsten-halogen lamps have very hot surfaces and can explode if these surfaces become contaminated. In addition, they may require spectral filters to reject out-of-band radiation, and forced-air cooling to reject excess heat. The forced-air cooling is a problem itself because fans induce vibration into the optical system and may cause mischief with optical measurements. Thus, what is needed is a way to make use of light sources for collimators that are more efficient, not prone to explosion, and do not require complex cooling solutions.